Previously we found that the adapter protein SHN3 had a major effect to suppress bone formation by osteoblasts in vivo by inhibiting the activity of the mitogen activated protein kinase (MAPK) ERK. We also identified that WNTs have a previously unidentified activity to activate this SHN3/ERK pathway. Here we seek to make critical extensions of these observations by determining how SHN3 inhibits ERK and how this pathway influences models of skeletal disease in vivo. This area of study is significant as osteoporosis is a highly prevalent disorder of low bone mass and skeletal fragility. Half of all women will experience of an osteoporotic fracture during their lifetime, with each of these fractures incurring significant morbidity and mortality. Based on the profound ability of SHN3 to suppress bone formation, building a better understanding of this pathway will yield new methods to control the activity of osteoblasts to form bone, potentially offering approaches to treat osteoporosis and other disorders of low bone mass. The three overlapping areas to be investigated in this project are: 1. Determine the contribution of SHN3 to skeletal disease models while we have established that SHN3 strongly suppresses bone formation in mice at baseline, the specific contribution of SHN3 to murine models of post-menopausal osteoporosis and skeletal fracture are unknown and will be studied in SHN3-deficient mice. 2. Determine how SHN3 inhibits the ERK pathway in osteoblasts While we have established that SHN3 acts in osteoblasts primarily by suppressing the activity of ERK, the relevant ERK substrates and how SHN3 shapes the kinetics of the ERK response are unknown and will be studied using phosphoproteomics and a quantitative ERK activity biosensor. Determining the relevant substrates of ERK may identify downstream effectors of the SHN3/ERK pathway that are themselves amenable to therapeutic targeting. Additionally, the mechanism for the activation of the ERK/SHN3 pathway by WNTs is unknown, and this will be addressed by a focused screen to determine the MAP3K required for proximal activation of this pathway. 3. Determine if chromatin acts as a scaffold to orchestrate the activity of the SHN3/ERK complex in osteoblasts. Both SHN3 and ERK have previously been identified to bind DNA, raising the possibility that regulation of ERK activity by SHN3 occurs in chromatin bound complexes. In this aim, a complimentary series of chromatin immunoprecipitation-sequencing experiments will be utilized to explore if chromatin orchestrates the function of the SHN3/ERK pathway.